For example, Japanese Non-examined Utility Model Publication No. Sho 62-15662 (Japanese Examined Utility Model Publication No. Hei 4-14645) has disclosed, as a pilot valve to be used for controlling actuation of hydraulic equipment, a pilot valve comprising a valve body, a push rod movably attached to the valve body, and a spool movably attached to a spool hole of the valve body. The valve body is provided with a spring chamber. First and second spring members are provided in the spring chamber. A spring bearing member is relatively movably attached to the spool. The first spring member is provided between the valve body and the spring bearing member to elastically bias the push rod upward through the spring bearing member. The second spring member is provided between the spring bearing member and the spool to elastically bias the spool in a direction apart from the push rod. The valve body is further provided with an operation lever. When the operation lever is turned, the push rod is pressed against elastic force of the first spring member. The spool has first and second spool portions which are provided at a constant interval in an axial direction. A notch is provided between the first and second spool portions. An outside diameter of the first spool portion is a little smaller than that of the second spool portion. A pressure receiving area of a pressure receiving part of the first spool portion is set smaller than that of a pressure receiving part of the second spool portion.
In such a pilot valve, a control lever is held in a neutral position when the operation lever is not manipulated. When the control lever is set in the neutral position, the push rod is held in a projecting position by action of the first spring member, and the spool is held in a non-working position by action of the second spring member. Consequently, the first spool portion of the spool blocks communication of a primary pressure side passage with a secondary pressure side passage, while the notch of the spool causes the secondary pressure side passage to communicate with a tank passage so that a fluid in the secondary pressure side passage flows into the tank passage. When the operation lever is turned to press the push rod in such a state, the spool is moved downward by the action of the second spring member and the notch of the spool causes the primary pressure side passage to communicate with the secondary pressure side passage so that a pressure fluid is fed from the primary pressure side passage to the secondary pressure side passage; The pressure fluid flowing in the secondary pressure side passage acts on the pressure receiving parts of the first and second spool portions so that the spool is biased upward depending on a pressure difference acting on the pressure receiving parts. Thus, the spool is held in a position where the elastic force generated by the second spring member is balanced with the pressure generated by the pressure fluid in the secondary pressure side passage the pressure generated by the pressure fluid depending on a difference in a pressure receiving area).
However, such a pilot valve has the following problems to be solved. More specifically, when the operation lever is manipulated to move the spool from a non-working position, the pressure fluid flows from the primary pressure side passage to the secondary pressure side passage. At this time, if the spool is moved downward to feed the pressure fluid from the primary pressure side passage to the secondary pressure side passage, the pressure of the secondary pressure side passage is raised. Consequently, the spool is about to be moved upward against elastic force of the second spring member. When the spool is thus moved somewhat upward, a part of the fluid in the secondary pressure side passage flows in a tank line so that the pressure of the secondary pressure side passage is dropped. Consequently, the spool is moved downward by means of the second spring member, and the pressure of the secondary pressure side passage is raised again by the pressure fluid flowing from the primary pressure side passage. In such a control state, the spool of the pilot valve easily causes a vibration phenomenon, that is, a so-called hunting phenomenon so that actuation of the pilot valve becomes unstable.
In order to prevent such a hunting phenomenon, Japanese Non-examined Utility Model Publication No. Hei 3-39602 has disclosed an improved pilot valve. In the improved pilot valve, a stepped piston is movably attached to one of ends of a spool and is supported on a part of a valve body. The stepped piston is attached to the spool to form a pressure chamber and a damper chamber on the other end. The pressure chamber is caused to communicate with a secondary pressure side passage, and the damper chamber is caused to communicate with a tank line. In such a pilot valve, the damper effect of the damper chamber can prevent the above-mentioned hunting phenomenon of the spool.
In the improved pilot valve, however, the stepped piston and the like are required Consequently, there are drawbacks that a structure is complicated and the number of parts is also increased. Furthermore, the damper chamber is formed to be built in the other end of the spool. Therefore, it is hard to keep a large capacity for the damper chamber. Consequently, good damper effects cannot be obtained.
It is an object of the present invention to provide a pilot valve in which great damper effects can be obtained with a comparatively simple structure and a hunting phenomenon can fully be prevented.